Conventional magnetic recording elements that are used for recording sounds or images are generally opaque to visible light regardless of the nature of the magnetic particles used in such elements. For example, motion picture films often are provided with a magnetic sound track which generally is opaque and does not cover that portion of the film used in the projection of images.
Canadian Patent 686,172 shows that a magnetic recording layer may be transparent to visible light when it contains low concentrations of magnetizable particles. According to this patent, such a layer is coated over a layer containing descriptive material which allows a user to simultaneously hear and see certain subject matter. However, this patent points out that the electromagnetic characteristics, i.e., the magnetic recording and reproducing characteristics, of such a layer are inferior to those of conventional magnetic layers as a result of the very low concentration of magnetizable particles.
U.S. Pat. No. 3,782,947 discloses a photographic product which carries magnetic particles distributed across the image area of the product. The particle distribution and sizes are so designed that the composite granularities of the photographic and magnetic recording media are such that the magnetic distribution is essentially transparent in a photographic sense. According to this patent, the photographic image can be viewed via the magnetic distribution and the magnetic distribution can be employed for recording and playing back information.
U.S. Pat. No. 4,279,945 discloses a process of preparing magnetic recording elements containing a transparent recording layer. According to this patent, the magnetic recording and reproducing characteristics obtained are comparable to conventional opaque magnetic layers without the need for matching the granularity of a magnetic medium to that of photographic medium. However, the process requires that the layer containing magnetic particles be treated using one or both of the following process steps, (1) compacting the layer while it is in a malleable state to reduce its thickness (e.g., calendaring), or (2) imbibing into the layer a substantially transparent liquid having a refractive index that is substantially the same as that of the binder.
Elements of the type described in the above-cited patents have not achieved widespread commercial success for various reasons. For example, the elements described in U.S. Pat. No. 4,279,945, as indicated by the figure therein, are substantially opaque at wavelengths less than about 500 nm and thus are not useful in color films. Further, the disclosed process requires that the magnetic recording layer be calendared while it is in a malleable state and/or that a transparent liquid be imbibed into the magnetic recording layer. On the other hand, U.S. Pat. No. 3,782,947 contemplates coating a dispersion containing magnetic particles onto a film base. However, the quantity of solvent required in such a process is unattractive from both an economic and environmental standpoint. Additionally, in continuous wide web coating techniques adapted for commercial manufacturing operations, solvent attack on the film base can render the element unusable, resulting in unacceptable manufacturing inefficiencies and excessive costs. Moreover, it is difficult to prepare magnetic recording layers in such a coating process having a thickness of less than about 5 microns.
U.S. Pat. No. 4,990,276 discloses a dispersion of magnetic particles, a dialkylester of phthalic acid and a dispersing agent. U.S. Patent No. 5,254,449 discloses the use of such a dispersion to provide a substantially transparent magnetic recording layer in the preparation of a novel photographic element. The process described in this patent calls for dilution of the dispersion with a binder in an organic solvent such as a mixture of methylene chloride and methanol, to form a casting composition. The casting composition is then extruded under pressure onto a photographic support.
Most magnetic recording layers which are part of imaging elements are coated from organic solvents. However, for environmental and cost reasons, it is desirable to eliminate the use of organic solvents in such applications. U.S. Pat. No. 5,147,768 discloses a method of coating an aqueous magnetic dispersion using gelatin as a binder. Copending, commonly assigned Nair U.S. patent application Ser. No. 08/229,267 et al., filed Apr. 18, 1994, now U.S. Pat. No. 5,457,012, describes the use of certain relatively low molecular weight polymeric dispersants to achieve stable aqueous magnetic dispersions from which transparent magnetic layers can be coated over imaging elements.
In the preparation of magnetic recording layers for photographic and other imaging elements it is important that the magnetic layer be transparent and free of haze in a photographic sense. This is a challenge when a very thin (e.g., &lt;5 microns) layer is coated from an aqueous dispersion containing only a small amount of a hydrophilic binder such as gelatin. Even if a dispersion is colloidally stable prior to the coating operation, during drying of the wet coated layer the vertical force component that acts on the magnetic particles as a result of the layer drying down can cause the particles to flocculate resulting in a hazy coating with increased grain.
It is known to use synthetic polymers to increase the viscosity of aqueous gelatin solutions in multilayer coatings for rheology control. It is known, e.g., to incorporate polymers containing acid groups such as carboxyl, sulfonate or sulfate groups into coating solutions to increase their viscosity for multilayer photographic coatings. For example, U.S. Pat. No. 3,022,172 discloses sulfonates of vinyl, allyl, styrene or alkyl benzene compounds to increase the viscosity of gelatin coating solutions. Further, Photographic Science & Engineering Vol. 14, pages 178-183 discloses that ammonium salts of maleic anhydride and methyl vinyl ether, polystyrene sulfonate, poly vinyl ammonium phthalate, dextran sodium sulfate etc., can be employed as viscosity increasing agents for gelatin. U.S. Pat. No. 3,655,407 discloses acrylic acid/alkyl acrylate copolymers to increase the viscosity of gelatin solutions. Additionally U.S. Pat. No. 4,166,050 and DD 213,768 disclose maleic anhydride copolymers as viscosifiers for gelatin solutions. DE patent 276,243 suggests the use of polymers containing mixed carboxylate sulfonate groups for viscosifying gelatin solutions and increased robustness to pH changes. Similarly, DE 4,034,871 discloses copolymers of maleic anhydride having pendent sulfonic acid groups.
Polysaccharides containing anionic moieties have also been disclosed as viscosifiers for gelatin solutions. Naturally occurring polysaccharides, like carageenan, have been disclosed in U.S. Pat. No. 3,250,620. Furthermore, synthetically modified polysaccharides containing anionic moieties have been disclosed as viscosifiers for gelatin solutions. For example, U.S. Pat. No. 3,335,128 discloses cellulose sulfate with mixed cations. U.S. Pat. No. 2,767,410 discloses polysaccharides where 50% of the hydroxyl groups are acetylated or sulfated. DE 3,914,947 discloses sulfoethyl substituted cellulose.
Anionic synthetic polymers and polysaccharides have also been disclosed for use in the preparation of silver halide photographic emulsions. These can be used for several purposes. U.S. Pat. Nos. 2,772,165; 3,241,969 and 3,341,333 disclose anionic polymers acting either alone or in combination with other materials to coagulate and precipitate the emulsion. GB 1,064,215 discloses the use of sulfoalkylated polysaccharides to increase the covering power of the coated emulsion. Yet another application is disclosed in U.S. Pat. No. 3,811,897 to reduce the sedimentation of emulsions.
U.S. Pat. No. 5,147,768 discloses the use of various hydrophilic polymers, including polyacrylic acid, as possible replacements for gelatin as the hydrophilic film forming binder in aqueous magnetic coatings. There is no suggestion, however, to use any particular type of hydrophilic polymer at any certain percentage in combination with gelatin to control the haze of the resulting coating and to yield transparent magnetic coatings of improved optical quality.
The above cited patents disclosing the use of viscosifying coating aids generally relate to the improvement of multilayer coating uniformity, primarily by changing the rheological properties of the coating melt of the bottom-most layer. There is no suggestion, however, to use such materials in a transparent magnetic layer in order to obtain haze-free layers where the problem to be solved is one of reducing the mobility of the magnetic particles during the coating and drying process. Generally, such layers are coated as the top-most layer in a coating pack. We have found that the use of polyelectrolytes can solve the problem by increasing the viscosity of the continuous phase thereby improving the stability of the wet layer that is laid down on the support.